A. Field of the Invention
The present invention relates to a basic structure element forming a magnetic memory or a magnetic sensor and a device using the element. More specifically, the invention relates to an element that controls electron spin injection to form a magnetic random access memory capable of multi-valued data recording, and a device using the element.
B. Description of the Related Art
In recent years, a giant magneto-resistance (GMR) effect element made up of layers arranged as a ferromagnetic layer/a nonmagnetic metal layer/a ferromagnetic layer and a tunnel magneto-resistance (TMR) effect element made up of layers arranged as a ferromagnetic layer/an insulator layer/a ferromagnetic layer have been developed with an expectation of application to a new magnetic sensor or a magnetic memory (MRAM: Magnetic Random Access Memory). FIG. 8A is a view showing a basic structure of an MRAM. As shown in FIG. 8A, in an MRAM, bit lines 52 and word lines 56 are arranged in a matrix-like form with element 50 such as a TMR magnetic memory element disposed at each of intersections of bit lines 52 and word lines 56. An example of an arrangement of TMR magnetic memory element 50 driven by a magnetic field produced by a current is as shown in a schematic cross sectional view of FIG. 8B. In FIG. 8B, the arrangement is shown as that in the case in which the coercive force (Hc) of second ferromagnetic layer 55 is higher than that of a first ferromagnetic layer 53. When writing logical information in the TMR element, the writing is carried out by letting a current flow in an additionally provided writing word line 57 to produce a magnetic field. By adjusting the direction and the strength of a magnetic field produced at this time, the direction of magnetization of first ferromagnetic layer 53 and that of second ferromagnetic layer 55 are made in parallel or in anti-parallel with each other, and information of “1” or “0” is stored. Read-out of stored information is carried out by letting a current flow from word line 56 to bit line 52 and reading a resistance value of the element appeared by the TMR effect when the current passes through insulation layer 54. In this way, logical information can be recorded into and regenerated from the TMR element. With the arrangement, however, only the binary data of “1” and “0”, i.e., information of only 1 bit can be recorded and regenerated in one memory cell. This is insufficient for the requirement for high-density recording and regeneration in the future. Furthermore, magnetic field production by means of a current requires large electric power consumption for writing information.
As a means of solving the problem, a proposal was made in which information writing is not carried out by a magnetic field produced by a current, but is instead carried out by direct injection of spin-polarized electrons. At present, attention has been focused on this proposal (see J. A. Katine et al., “Current-Driven Magnetization Reversal and Spin-Wave Excitations in Co/Cu/Co Pillars,” Physical Review Letters, Vol. 84, No. 14, pp. 3149-3152 (2000), for example).
FIGS. 9A and 9B are schematic cross sectional views that explain an example of an arrangement and an operation principle of a related magnetic memory element that carries out recording and regeneration of binary information data by using magnetization reversal due to electron spin injection (see US2004/0165425 A1, for example). The memory element is made up of first ferromagnetic fixed layer 131, first isolation layer 141, first ferromagnetic free layer 151, second isolation layer 142 and second ferromagnetic fixed layer 132. FIG. 9A also shows states of electron spins and magnetization in ferromagnetic layers when electrons are made to flow in the element from second ferromagnetic fixed layer 132 toward first ferromagnetic fixed layer 131. In the figure, an open arrow represents the direction of magnetization in each ferromagnetic layer, an arrow with a small circle represents the direction of the spin of an electron and a thin arrow without circle represents the direction in which electrons flow (this is similar in the following drawings). Moreover, in the following explanations, the direction of magnetization is to be referred to as “rightward” or “leftward” in correspondence with the direction of the open arrow in the figure. This is similar about the direction of the electron spin. First, the spins of electrons passing second ferromagnetic fixed layer 132 are aligned in the direction of the magnetization of second ferromagnetic fixed layer 132 by interactions (s-d interactions) with spins of magnetic metal atoms in second ferromagnetic fixed layer 132 (polarization of spins is caused). The electrons with polarized spins flowing into first ferromagnetic free layer 151 cause the spin angular momentum of the electrons to be transferred to the magnetic metal atoms in first ferromagnetic free layer 151 to affect the magnetization of first ferromagnetic free layer 151. The direction of magnetization of first ferromagnetic fixed layer 131 is opposite to the direction of magnetization of second ferromagnetic fixed layer 132. Therefore, at the interface at which electrons flow into first ferromagnetic fixed layer 131, electrons with rightward spins, spins in the same direction as the direction of the magnetization of second ferromagnetic fixed layer 132, are reflected. The rightward spins possessed by the reflected electrons also affect the magnetization of first ferromagnetic free layer 151. Namely, electrons with rightward spins, spins in the same direction as the direction of the magnetization of second ferromagnetic fixed layer 132, affect the magnetization of first ferromagnetic free layer 151 twice, by which substantially two times of writing action can be obtained. As a result, it is considered that writing in first ferromagnetic free layer 151 can be carried out with a current smaller than that in a related system with a magnetic field produced by a current.
Reversal of the direction of the magnetization in first ferromagnetic free layer 151 is carried out by letting electrons flow from first ferromagnetic fixed layer 131 toward second ferromagnetic fixed layer 132. The states of electron spins and magnetization in the ferromagnetic layers at this time are shown in FIG. 9B. In this case, the electron spins are affected by the magnetization in first ferromagnetic fixed layer 131 to be made directed leftward on the paper. The electrons with the spins in first ferromagnetic free layer 151 affect the magnetization thereof. Furthermore, the electrons with leftward spins are reflected at the interface with second ferromagnetic fixed layer 132 magnetized in the direction opposite to that of the spins to affect the magnetization of first ferromagnetic free layer 151 again. Thus, an arrangement is proposed in which, with two ferromagnetic fixed layers prepared with magnetization thereof being opposite to each other, the direction of a current made to flow in the element is changed to thereby write binary information into first ferromagnetic free layer 151.
Moreover, an arrangement is proposed in which a number of elements of this kind are stacked to thereby carry out multi-valued data recording (see paragraphs [0142] and [0143] in US2004/0165425 A1, for example). FIG. 10 is a schematic cross sectional view for explaining an arrangement of a spin injection magnetization reversal element carrying out multi-valued data recording, which is disclosed in US2004/0165425 A1. The element, using element terms used in this specification, includes first ferromagnetic fixed layer 131, first isolation layer 141, first ferromagnetic free layer 151, second isolation layer 142, second ferromagnetic fixed layer 132, third isolation layer 143, second ferromagnetic free layer 152, fourth isolation layer 144 and third ferromagnetic fixed layer 133. In the figure, a layer with two open arrows indicating their respective directions as being opposite to each other shows that the direction of magnetization in the layer is variable. The element basically has an arrangement in which two elements, each shown in FIG. 9, are stacked, though they share second ferromagnetic fixed layer 132. It is considered that changes in magnetic materials and film thicknesses of first ferromagnetic free layer 151 and second ferromagnetic free layer 152 change a critical current density for reversing the direction of magnetization (hereinafter referred to as a magnetization reversal current density) in each of the layers to thereby enable multi-valued data recording.
The above-explained arrangement is based on the concept that multi-valued data recording is carried out by stacking a plurality of elements each carrying out binary data recording. Therefore, for each layer of the ferromagnetic free layers recording magnetization reversal, at least one layer of the ferromagnetic fixed layer must be disposed. This increases the number of the ferromagnetic fixed layers as the number of multi-valued data increases. The ferromagnetic fixed layer, however, is required to have a sufficiently large thickness for fixing magnetization without easily causing variation. As a result, in the case of providing a plurality of ferromagnetic fixed layers, element resistance is to be increased to increase power consumption when the element is in operation.
Moreover, since multi-valued information is recorded by varying the magnetization reversal current densities in a plurality of ferromagnetic free layers, a design guideline becomes important for setting the magnetization reversal current densities at values different from one another. For such a design guideline for the ferromagnetic free layers, a proposal of varying the thicknesses of respective ferromagnetic free layers is presented. This, however, is not a preferable measure because an increase in the layer thickness causes an increase in a resistance value which results in an increase in power consumption. In addition, in the present situation, no specific criterion is presented about determination of a magnetic material when a ferromagnetic free layer is designed.
The present invention was made by giving attention to the above-explained points, and it is an object of the present invention to provide an arrangement that enables multi-valued data recording with an increase in resistance in the element suppressed. A further object of the invention is to provide criteria for determining magnetic materials and the compositions thereof by which a plurality of ferromagnetic free layers with their respective values of critical currents for magnetization reversal distinctly different from one another can be formed without varying their respective thicknesses.